API Testing: What It Is, Why It’s Important & How to Do It

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Anna Fitzgerald
Anna Fitzgerald


The global API testing market is projected to reach 1.8 billion USD by 2026, up from 641.6 Million in 2020, according to a report by Global Industry Analysts Inc.

Individual learning about API testing

This market growth corresponds to the rise in cloud applications and interconnected platforms that require application programming interfaces (APIs). More APIs mean more testing to determine that they meet expectations for functionality, reliability, performance, and security. Without this testing, an API may fail to perform as expected, which can impact many softwares and services.

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Software application development is dominated by the three-layered architecture approach, which is an architecture made up of a presentation layer, business logic layer, and database layer.

three layer architecture in software application includes presentation, business, and data access layer

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The business logic layer contains the core functionality, or components, of the application. It must take information inputted by the user from the presentation layer, query the database layer and transform that data according to the business logic, and present the results back to the user via the presentation layer. But the business logic layer must also communicate with other applications as well as human users. To do so, it uses an API.

API wraps around business logic layer to enable application to communicate with API clients

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So API testing is designed to validate the business logic as well as the performance, security, and other aspects of the application. API testing does not focus on the individual components of an application, like unit testing, or the look and feel of the application, like user interface (UI) testing, but on what the application does.

Diagram showing that API tests focus on API while unit tests focused on individual components in business logic layer

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To assess whether the API works like it’s supposed to, you need to run multiple tests. Let’s take a look at the most common types below.

Types of API Testing

There are multiple API testing approaches, including:

  • Functional testing. These API tests are designed to check that an API returns the right response for a given request.
  • Load testing. This type of API test gauges how an API handles a large volume of requests over a short period.
  • Runtime and error detection testing. These API tests are designed to evaluate the actual running of the API and typically focus on monitoring, execution errors, resource leaks, or error detection.
  • Security testing. These tests assess how an API responds to and resists cyberattacks.
  • Penetration Testing. Penetration tests involve users with limited API knowledge trying to attack the API, which enables testers to assess the threat vector from an outside perspective.
  • Fuzz Testing. This type of API test sends a large number of randomized requests to see if your API responds with errors, processes any of these inputs incorrectly, or crashes.
  • Validation testing. Validation tests are run late in the testing stage to verify the behavior and efficiency of the API.

For a more in-depth overview of what API testing is, including the different types of API testing as well as the benefits and challenges, check out this video:

API testing is also important because it offers several advantages over other types of testing, like unit and UI testing.

For example, unit tests are designed to verify the functionality of individual components within a single application whereas API tests are designed to verify that all system components function as intended. This broader test coverage makes it easier to identify any bugs in the unit, database, and server levels.

API tests are also faster to run and more isolated than UI tests, which makes it quicker and easier to identify and resolve bugs. According to data from Andersen Lab, a UI test runs for approximately seven minutes while an API test runs for 12 seconds. Meaning, an API test is about 35 times faster than a UI test.

Perhaps most importantly, API testing allows developer operations, quality assurance, development, and other teams to begin testing an application’s core functionality before the user interface is ready. This enables them to identify any errors or weaknesses early on in the development process. If identified later in the process, these errors and weaknesses in the build can be costly to fix, requiring large amounts of code to be rewritten and significantly delaying the product’s release.

Now that we understand some of the benefits of API testing, let’s walk through how to perform it.

1. Review the API specification.

Before you start testing, you need to first understand the purpose of the API, how the API functions, and what results to expect when using the API. To do so, you can review the API specification.

For example, if you’re testing an HTTP API, then you can review the OpenAPI specification, which defines a standard, programming language-agnostic interface description for HTTP APIs. This specification details all the HTTP API’s objects, values, and parameters, how the objects are called, what each object does, and how they can be used together. The section below details how the Request Body Object is supposed to function, what its fixed fields are, what to expect when using this function, and request body examples.

Request Body Objection section in the OpenAPI specification

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2. Determine API testing requirements.

Next, you need to determine the testing requirements of the API. This will require you to understand the API’s target consumer, its features and functions, and the workflow of the application as well as the aspects, priorities, and problems you’re testing for. For example, maybe you want to verify that HTTP headers are as expected or that a response is received within a reasonable period of time, as defined by the test plan.

You’ll also need to know the desired output — which could be a 2XX HTTP status code or a JSON object, for example.

3. Define input parameters.

Before calling an API, you must define input parameters. These parameters pass needed information to the API to enable it to perform its function and therefore are necessary for determining whether the API performs as expected. For example, a REST API can accept header, query, and rest body parameters, among other input parameter types.

Before beginning API testing, it’s important to plan out all possible input combinations.

4. Create positive and negative tests.

To assess whether an API functions as expected, you should run a combination of positive and negative tests.

  • Positive tests are designed to check the basic functionality of the API using required parameters as well as extra functionality using optional parameters.
  • Negative tests are designed to check how the API responds to prohibited operations using valid and invalid user input, like trying to input a username that already exists or a username that is null.

5. Select an API testing tool.

Now you’re ready to select an API testing tool that can help automate or simplify the API testing process. When evaluating different API testing tools, it’s important to know what kind of API you’ll be testing, what kind of tests you’ll be running, and what your budget is.

If you’re looking for recommendations, check out 10 Best API Testing Tools for Building Functional, Secure Applications.

For example, Postman is a great option for testing REST APIs and you can get started for free. For a complete walkthrough of how to test an API using Postman, checkout this video:

API Automation and the Future

With an increasing number of services relying on hundreds of interconnected APIs, API testing is expected to continue to grow in importance. API automation, through robust API testing, is the key to maintaining connectivity in the digital economy and enabling faster product releases. It allows for accurate, repeatable, and efficient testing processes, crucial to keeping up with the speed and scope of today's software development.



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